Add cacti as a submodule

[Andful]

It seems more elegant to add cacti as a submodule for the following reasons:

• it is easier to track what version of cacti that is used
• If we want tweaked dependencies, a fork can be made with its own history
• reduces size of the repository

Furthermore, currently the folder contains compiled binaries that I think were not intended to be pushed.

That said, an additional attention is needed when cloning the repository. Users would have to run git clone --recursive to clone the submodules when cloning the main repo.

Feel free to close the pull request if you deem the current method to be fine.

[asyms]

Unfortunately we had to modify some cacti files, so this isn't possible. In the future we will move to another memory modeling tool.